Man Haron Monis was so desperate to be accepted into any group or cause that the Lindt Cafe siege might never have occurred if his efforts to join the Rebels bikie gang had succeeded, a terrorism expert says.

Dr Clarke Jones from the Australian Intervention Support Hub told the ongoing inquest into the siege that he did not believe Monis was correctly categorised as a terrorist despite his claimed affiliation to Islamic State.

"He was willing to join whatever he could join – Hizb ut-Tahrir ... he made moves to try and assist Australian intel agencies which regarded him as a serial pest," Dr Jones said, while participating in a panel session with two other terrorism experts.

"I think his wanting to join something is evident with his membership of the Rebels Outlaw Motorcycle Club. I wonder whether, if he had been accepted, would we be here today?"

The ageing Le Salle 12gauge sawn-off shotgun used by Monis during the siege. Photo: Department of Justice

He told the inquest that the evidence suggested that Monis' "narcissism" and mental health issues played a greater role in the siege than terrorist ideology.

This had simply been an "external justification" for his own "internal processes".

But Professor Greg Barton of Deakin University said he believed the siege had many of the objective features of a terrorist act, including that it was in keeping with the IS strategy of targeting those with mental health issues.

"Al-Qaeda tended to be careful about weeding people out with mental instability," Professor Baton said.

"IS is different – it gets people with no connection and says, 'You do something in our name and we'll recognise it'. It goes after damaged goods."

He said that as a result of this, the community could expect more "lone wolf" attacks such as the recent Amsterdam-Paris train attack.

"We should now expect many more unpredictable attacks in the future," he said.

He also noted that Monis had been highly critical of the Australian government for eight years prior to the attack.

But Professor Shanahan said these attacks had been characterised by confusion and contradiction.

"On the one hand he's praising Islamic State and on the other he doesn't know who the leader is," he said.

"Can you believe anything he really says? He says one thing and then turns around 180 degrees the next moment."